The present invention relates to a method for controlling the brake pressure of a vehicle as a function of the deviation of the vehicle wheels relative to a prescribed desired slip.
German patent document DE 40 30 724 A1, discloses a system of this type, in which the desired slip is determined not only as a function of the vehicle speed but also as a function of the cornering forces. In this case, the specified slip is determined from longitudinal forces and lateral forces on the wheel which have been determined. In addition to the wheel speeds, the yaw velocity and the steering angle are required for this purpose as additional measured variables. The yaw behavior of the vehicle is then controlled on the basis of the desired yaw rate, derived from the measured steering angle, and the actually measured yaw velocity. A specified brake pressure is then determined for each wheel from the system deviation between the desired slip and the actual slip; and the brake pressure is controlled.
A method is disclosed in German patent document DE 40 30 704 A1 by means of which a desired slip value is determined from the measured variables of steering angle, yaw velocity, wheel speeds and brake pressure by using a vehicle model. In this case, the calculation of the desired slip by means of a vehicle model and an assigned controller is computationally intensive and likewise requires a high outlay on sensors.
In both of these methods, it is necessary to know the yaw behavior or the slip angle of the wheels from the specification of a path curve for the vehicle, which in turn is determined from the steering angle and yaw rate. The cornering forces are determined in this case from the vehicle model, and not from the relationships between the wheel and road. The yaw behavior is controlled on the basis of a driving condition which has arisen whenever the yaw velocity deviates beyond a measure of tolerance from the desired yaw rate derived from the steering angle. The specified desired slip for wheel brakes is influenced in accordance with a stabilization of the vehicle in order to control the yaw behavior of the vehicle, and thus to stabilize the driving condition.
In contrast, in the case of the conventional antilock braking system, the brake pressure is controlled so that, for example, the maximum braking force between the wheel and road is determined by the specification of the desired slip. In this case, the desired slip is specified via an evaluation of the wheel speeds. Such methods do not, however, take account of the lateral forces on the wheel. A stable driving condition is not guaranteed in the case of cornering, since it is not always possible at maximum braking force to apply an adequate lateral force to the tire. Depending on the conditions of the vehicle, this can lead to understeering or oversteering.
The object of the present invention is to configure a brake pressure control by means of specified desired slip so as to guarantee stable or neutral handling. Unlike conventional antilock systems, this is to be performed without additional sensors and without a high computational outlay.
This object is achieved by the brake pressure control process according to the invention, in which the desired slip is determined from the wheel speeds such that the cornering forces occurring between wheel and roadway and the adhesion conditions occurring, ensure stable handling. In one embodiment of the invention, the desired slip is maximized within the range limits afforded by the requirements placed on the cornering forces.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.